On Tue, Nov 6, 2018 at 10:30 AM, Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> We take global fiq->waitq.lock every time, when we are
> in this function, but interrupted requests are just small
> subset of all requests. This patch optimizes request_end()
> and makes it to take the lock when it's really needed.
>
> queue_interrupt() needs small change for that. After req
> is linked to interrupt list, we do smp_mb() and check
> for FR_FINISHED again. In case of FR_FINISHED bit has
> appeared, we remove req and leave the function:
>
> CPU 0 CPU 1
> queue_interrupt() request_end()
>
> spin_lock(&fiq->waitq.lock)
>
>
> list_add_tail(&req->intr_entry, &fiq->interrupts) test_and_set_bit(FR_FINISHED, &req->flags)
> smp_mb() <memory barrier implied test_and_set_bit()>
> if (test_bit(FR_FINISHED, &req->flags)) if (!list_empty(&req->intr_entry))
>
> list_del_init(&req->intr_entry) spin_lock(&fiq->waitq.lock)
> list_del_init(&req->intr_entry)
>
> Check the change is visible in perf report:
>
> 1)Firstly mount fusexmp_fh:
> $fuse/example/.libs/fusexmp_fh mnt
>
> 2)Run test doing
> futimes(fd, tv1);
> futimes(fd, tv2);
> in many threads endlessly.
>
> 3)perf record -g (all the system load)
>
> Without the patch in request_end() we spend 62.58% of do_write() time:
> (= 12.58 / 20.10 * 100%)
>
> 55,22% entry_SYSCALL_64
> 20,10% do_writev
> ...
> 18,08% fuse_dev_do_write
> 12,58% request_end
> 10,08% __wake_up_common_lock
> 1,97% queued_spin_lock_slowpath
> 1,31% fuse_copy_args
> 1,04% fuse_copy_one
> 0,85% queued_spin_lock_slowpath
>
> With the patch, the perf report becomes better, and only 58.16%
> of do_write() time we spend in request_end():
>
> 54,15% entry_SYSCALL_64
> 18,24% do_writev
> ...
> 16,25% fuse_dev_do_write
> 10,61% request_end
> 10,25% __wake_up_common_lock
> 1,34% fuse_copy_args
> 1,06% fuse_copy_one
> 0,88% queued_spin_lock_slowpath
>
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> ---
> fs/fuse/dev.c | 30 ++++++++++++++++++++++--------
> 1 file changed, 22 insertions(+), 8 deletions(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 7705f75c77a3..391498e680ec 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -427,10 +427,16 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
>
> if (test_and_set_bit(FR_FINISHED, &req->flags))
> goto put_request;
> -
> - spin_lock(&fiq->waitq.lock);
> - list_del_init(&req->intr_entry);
> - spin_unlock(&fiq->waitq.lock);
> + /*
> + * test_and_set_bit() implies smp_mb() between bit
> + * changing and below intr_entry check. Pairs with
> + * smp_mb() from queue_interrupt().
> + */
> + if (!list_empty(&req->intr_entry)) {
> + spin_lock(&fiq->waitq.lock);
> + list_del_init(&req->intr_entry);
> + spin_unlock(&fiq->waitq.lock);
> + }
> WARN_ON(test_bit(FR_PENDING, &req->flags));
> WARN_ON(test_bit(FR_SENT, &req->flags));
> if (test_bit(FR_BACKGROUND, &req->flags)) {
> @@ -470,13 +476,21 @@ static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
> {
> bool kill = false;
>
> - spin_lock(&fiq->waitq.lock);
> - if (test_bit(FR_FINISHED, &req->flags)) {
> - spin_unlock(&fiq->waitq.lock);
> + if (test_bit(FR_FINISHED, &req->flags))
The only case this test would make sense if this was a performance
sensitive path, which it's not.
> return;
> - }
> + spin_lock(&fiq->waitq.lock);
> if (list_empty(&req->intr_entry)) {
> list_add_tail(&req->intr_entry, &fiq->interrupts);
> + /*
> + * Pairs with smp_mb() implied by test_and_set_bit()
> + * from request_end().
> + */
> + smp_mb();
> + if (test_bit(FR_FINISHED, &req->flags)) {
> + list_del_init(&req->intr_entry);
> + spin_unlock(&fiq->waitq.lock);
> + return;
> + }
> wake_up_locked(&fiq->waitq);
> kill = true;
> }
>
